The present invention relates to ultrasonic transducers for inspecting materials having very low acoustic impedance, for example porous and fibrous materials.
The ultrasonic inspection of low acoustic impedance materials such as polyurethane foam or fibrous ceramics and others is a very difficult task for a variety of reasons. Ultrasonic inspection is usually carried out by means of piezoelectric transducers. The particular piezoelectric materials which are suitable for serving as active elements in ultrasonic transducers have an acoustic impedance which is much larger than the acoustic impedance of foam or of fibrous ceramic material. By way of example, lead-zirconate-titanate, a typical piezoelectric material, has an acoustic impedance which is almost 700 times the acoustic impedance of polyurethane foam, and about 500 times the acoustic impedance of fibrous silica ceramic. In other words, there is an inherent, significant mismatch in the acoustically active and generating material of the transducer on the one hand, and certain materials to be inspected on the other hand.
As such a transducer interfaces with low impedance material for purposes of transmitting thereto acoustic signals, most of the vibrations will be reflected back into the transducer, and very little energy will propagate into the material to be inspected. While a sufficiently strong inspection signal can be generated simply by driving the transducer with sufficient power, most of the electric energy applied to the transducer will remain therein and will be dissipated in some fashion. Accordingly, the transducer will ring so that short range echo signals returning to the transducer are readily obscured. Intensive damping of tranducers of available construction was found to be inadequate because it desensitizes the transducer for receiving echo signals to such an extent that only very strong echos can be detected.
The problem outlined above is compounded by the fact that transducers must be sufficiently broad banded for reasons of adequate resolution. Moreover, the transducers must have a sufficiently wide aperture to emit a relatively large wave front while capturing return echos over a sufficiently wide geometric range and area. It was found that conventional transducers vibrate in a variety of modes but only one mode, namely the mode oscillating in the direction normal to the interface with the object to be inspected, is of interest. Limiting the band width and/or providing for broad banded strong damping (to impede ringing) for eliminating the unwanted modes desensitizes, again, the transducer, and weaker echo signals will not be detected.
The problem is further compounded by the fact that porous and fibrous material attentuate high frequency acoustic signals to such an extent that the signal fails to penetrate sufficiently deep into the materials inspected. Lower frequencies have a better penetration than higher frequencies, but ringing is more pronounced at lower frequencies. As was mentioned above, such ringing tends to obscure echos at lower frequencies, particularly if the echos are weak. These problems and alternative attempts to solve them are discussed in a paper by me and another "Proceedings 10th Symposium on NDE," San Antonio, Tex., Apr. 23-25, 1975, published later in that year.
Upon considering the foregoing, it must be borne in mind that as long as piezoelectric transducers are to be used, the very high acoustic impedance mismatch with a porous or fibrous material is an inevitable constraint. Different piezoelectric materials may be discovered in the future but, broadly speaking, it cannot be expected that one will find always the suitable piezoelectric transducer material for each kind of material to be inspected. Additionally, the dependency of the penetration depth of ultrasonic vibrations on frequency is an inherent property. Thus, the detection of deep penetration echo signals makes mandatory the use of as low an inspection frequency as possible.
Considering these conditions as outlined above, it must readily be said that the ultrasonic inspection of construction parts made of porous or fibrous materials has not yet been adequately solved, and the difficulties encountered originate with basic properties of the materials involved.